Don't download identical APKs on update.

chrome/browser/android/webapk/webapk.proto

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package webapk;
 
 // Response after creating or updating a WebAPK.
 message WebApkResponse {
   // Package name to install WebAPK at.
   optional string package_name = 1;
 
   // Version code of the WebAPK.
   optional string version = 2;
 
-  // URL to download WebAPK.
+  // URL to download WebAPK. Note that this can be empty, and if it is, the
+  // client should not attempt to download or install anything. For example,
+  // this may be empty when the client requests an update even though no content
+  // has changed. The WAM server can then let the client know that it is
+  // up-to-date already and does not need to spend bandwidth re-downloading a
+  // WebAPK it already has.
   optional string signed_download_url = 3;
 
   // Unique id identifying session with WebAPK server.
   optional string token = 6;
 
   reserved 4, 5;
 }
 
 // Sent as part of request to create or update a WebAPK.
 message WebApk {
@@ skipping 47 matching lines @@
   // Murmur2 hash of the icon's bytes. There should not be any transformations
   // applied to the icon's bytes prior to taking the Murmur2 hash.
   optional string hash = 5;
 
   // Actual bytes of the image. This image may be re-encoded from the original
   // image and may not match the murmur2 hash field above.
   optional bytes image_data = 6;
 
   reserved 2 to 4;
 }